From Swimmers to Swarms: The Multi-scale Flow Structures of Vertical Migrations

Swarms exhibit inherently multiscale fluid dynamics, whereby interactions among swimmers generate collective flows that transcend the linear superposition of individual organism-induced flows. Evaluation of active transport pathways induced by the vertical migrations of mesozooplankton swarms has long been of relevance in Earth system science due to its potential role in the redistribution of carbon, nutrients, and oxygen in the upper ocean. In this study, we perform multi-scale velocimetry experiments to characterize how the hydrodynamic signature of individual copepods contributes to swarm-scale flows as a function of organism density and swarm orientation. We achieve this through in vivo lab experiments involving triggering and controlling the vertical migration of copepod swarms using phototactic stimulation. Our experimental setup enables us to capture both upward and downward migrations, as well as control the stratification profile of the water column. This way we obtain ecologically relevant calculations of kinetic energy, dissipation rates, mixing efficiency, and eddy diffusivity. Our results will be used to discretize the transport and mixing of fluid by migrating swarms in ocean models to evaluate their ecological relevance in the upper ocean.